It is known that while postal items are being automatically unstacked in a postal sorting machine, it can happen that two or more postal items can be unstacked together (i.e. bunched together in bunches of two or more), which means that they are conveyed in mutually overlapping manner. Overlapping bunched-together postal items cannot be automatically sorted correctly.
Known methods exist for detecting bunched-together postal items.
A first such method consists in mechanically rubbing both faces of a current postal item upstream from the unstacking in order to separate any other postal item overlapping the current item face-on-face.
A second method consists, at a plurality of measurement points along the path of the postal items, in measuring certain physical magnitudes of the postal items such as length, thickness, and height, in order to detect the presence of bunched-together postal items.
A third method consists in detecting the presence of bunched-together postal items by means of image analysis. In that third method, it is possible to form the image of a postal item seen from the front (that image generally being delivered by means of the optical reader for reading addresses), and the presence of any bunched-together postal items is detected by analyzing the top outline of the postal item, as described in Document FR 2 841 487.
It is also possible, using that third method, to form an image of the bottom edge of the postal item by means of a camera placed under the path of the postal item. Patent Document WO-03047773 describes another method of detecting bunched-together postal items. In that method, the postal items are conveyed, after unstacking, in series and on edge, and a camera is placed under the conveyor so as to form an image of the bottom side (bottom edge) of each postal item passing above the camera. The camera is a scanning linear array charge coupled device (CCD) camera (scanning transversely to the direction of movement of the postal item and thus transversely to the bottom edge (side) of the postal item). The image is a grayscale (GS) image. In that known method, the digital image is scanned row-by-row in the scanning direction (direction perpendicular to the direction of movement of the postal item) in order to measure the grayscale values on each row, and if, over all of the rows, it is detected that two or more maxima are spaced apart on each row and are repeated in a pre-recorded pattern over a set of rows in the image, it is deduced that bunched-together postal items are present.
The first method of detecting bunched-together postal items suffers from the drawback of generating large costs for being integrated into a postal sorting machine. In addition, the postal items can be damaged by the rubbing exerted on them.
The second method of detecting bunched-together postal items suffers from the drawback of not being adapted to a broad range of different postal items.
Analyzing outline using the third method can be unsatisfactory for detecting bunched-together postal item situations in which, when viewed from the front, one item is completely hidden by another postal item that is of larger size.